This invention relates in general to apparatus for working on sheet material and deals more particularly with a method and apparatus for accurately shifting or indexing sheet material over the supporting surface of a work table.
Automatically controlled instrument systems having instrument carriages which move in two coordinate directions over a work surface of a table are well known. Typically, such a system includes a carriage assembly which has a first carriage supported to traverse the table in one coordinate direction parallel to the longitudinal axis of the work surface and a second carriage mounted on the first one and which moves relatively to the first one and in another coordinate direction. Composite movement of both carriages allows an instrument mounted on the second carriage to be translated to any point over a region of the supporting surface traversed by the carriage assembly. Accurate positioning of the carriages and, consequently, of the instrument is achieved by numerical controls which operate either from an on-line data generator or from previously programmed data. Automatically controlled machines of the aforedescribed type may be provided with a wide variety of instruments which may, for example, include plotting pens or styluses, lightheads, tracking heads and cutting or drilling tools. Such known systems have also been provided which include a conveyor or a separate drive mechanism for shifting sheet material along a table surface so that a long strip of material can be worked upon in segments. Such conveyors or drive mechanisms have also been employed for loading material onto or unloading material from the work table.
The difficulty of working upon successive contiguous segments of a strip of material is that indexing movement of a strip, unless precisely controlled, will not permit continuous patterns to extend between the contiguous segments of the strip. Although continuity between segments may not be important in some applications, in others it is critical. In cutting patterns from long layups of sheet material, for example, if the patterns in adjacent segments overlap, even by small amounts, the resulting cut parts may be unsuitable for use. Further, if adjacent patterns cut in successive segments are spaced an unnecessary distance apart material is wasted.
At least one system has been provided wherein the aforesaid problem is solved by directly coupling the sheet material to the carriage so that the sheet material is precisely shifted relative to the work table and in one direction with the carriage. Such apparatus is illustrated and described in U.S. Pat. No. 3,844,461 to Robinson et al for Precise Indexing Apparatus and Method, issued Oct. 29, 1974 and assigned to the assignee of the present application. While such indexing apparatus is quite suitable for light duty operation as, for example, shifting plotting paper in a high resolution plotting system where a series of lines may extend continuously over several successive segments of a strip which is longer than the plotting table, additional problems are encountered when such a system is used for a heavy duty operation, as for shifting a layup of heavy fabric or the like relative to a cutting table. Such heavy duty apparatus generally requires an additional shifting mechanism such as a conveyor or other material shifting device for moving the layup or allowing it to be moved relative to the table. Normally, a relatively light duty drive motor may be used in a fabric cutting apparatus to move the carriage assembly, which carries the cutting mechanism, relative to the table. However, if the fabric layup is coupled directly to the carriage assembly the latter assembly must apply pulling force to move both the layup and the shifting mechanism or conveyor which supports it and requires a relatively heavy duty drive mechanism. A further problem is encountered in positioning a layup where the layup is directly coupled to a carriage assembly due to the inertia of the heavy load which must be started and stopped by the carriage assembly. The present invention is primarily concerned with this problem.
In a layup cutting apparatus of the aforedescribed general type the cutting instrument generally must pass through the layup and penetrate the layup supporting surface. It is also generally desirable that some means be provided for compressing the layup and firmly holding it in fixed position on a supporting surface while it is cut, so that all patterns cut from the layup will be substantially identical. Heretofore, bristle mats have been utilized quite successfully to provide zoned vacuum holddown whereby holddown force may be applied to the material locally, that is in the region in which the cutting mechanism is operating. The present invention is further concerned with improvements in such zoned vacuum holddown tables.
When a vacuum holddown table having a surface defined by bristles is used to hold sheet material such as fabric, in a layup cutting apparatus, threads, lint and small pieces of scrap material tend to accumulate between the bristles and reduce the holddown efficiency of the table. The present invention is also directed to this problem.